Lithium secondary battery with flake graphite negative electrode

ABSTRACT

A lithium secondary battery has a negative electrode structure, an electrolyte and a positive cathode structure, the negative electrode structure being of at least 40% by weight of natural flake graphite, having an L a -value of at least 300 nm and/or an L c -value in the range 50-150 nm and an L a -L c -ratio of at least 2, and the electrolyte having at least 10% by weight of propylene carbonate based on the weight of the solvent and salts of the electrolyte system. Natural flake graphite is compatible with propylene carbonate containing electrolytes, thereby forming stable battery configurations.

The present invention relates to a lithium secondary battery, inparticular a lithium secondary battery having a graphite negativeelectrode structure and comprising a non-aqueous electrolyte, and to amethod for the production thereof.

The use of lithium intercalation electrode structures and non-aqueouselectrolytes has allowed the development of electrochemical systemsbased on carbon for the negative electrode and transition metal oxidesfor the positive electrode. Such batteries, which are referred to as“rocking chair batteries” since the lithium ions are “rocked back andforth” between the electrodes, or as “lithium-ion batteries” since theactive lithium is always in its ionic form, display high energy densityand high cyclability, i.e. the system can be discharged and recharged alarge number of times.

For the negative electrode structure of such electrochemical systems,the electrode capacity, rate capability and cyclability are related tothe physico-chemical characteristics of the constituent carbons (cf.Ebner, W. et al.: Solid State Ionics, Vol. 69 (1994) pp. 238-56).Ordered carbon structures like graphite are usually referred to asoffering a higher reversible intercalation capacity, compared to themore disordered structures like coke. Graphite-based lithium-ionbatteries also have a higher operational voltage and they display a flatdischarge profile highly adapted for a large number of electronicapplications.

Graphite is a highly ordered carbon with a well defined structure. Thecarbon atoms are arranged in hexagonal rings in a two-dimensional array.The length of this array is defined as L_(a). The hexagonal ring-layersare stacked on top of each other, either in an ABAB-sequence, or, lessusually, in an ABCABC-sequence. The distance between two adjacentlayers, defined at d₀₀₂, is 3.354 Å, and the length of the stackedlayers along the stacking direction is defined as L_(c). Graphite has adensity of 2.26 g/cm³.

There are two types of graphite; natural graphite and artificialgraphite. Natural graphite is found in the earth's crust whereasartificial graphite is produced through heating of e.g. cokes orcarbonaceous gases up to 2500-3400° C.

Most commonly, the graphite used for negative electrodes in lithium ionbatteries is an artificial graphite. The L_(a) and L_(c) values thereofare usually larger than 100 nm and L_(a)/L_(c) ratio less than 2.Compared to artificial graphite, natural graphites, and in particularnatural flake graphites, display crystallites, which are longer andthinner. For the natural flake graphites, the L_(a)-value is typically 2to 10 times higher than the L_(c)-value. A typical natural flakegraphite has an L_(a)-value of 300 nm and an L_(c)-value of 50-100 nm.

A key feature for a negative electrode based on carbon is the initialirreversible loss which occurs during the first charge of the battery.This is due to an electrochemical reaction between the carbon negativeelectrode—or merely the lithium content thereof—and the lithiumsalt-containing organic electrolyte. During the reaction a passivatingfilm is formed on the anode, preventing it from further reacting withthe electrolyte.

Negative electrodes based on coke structures can function perfectly withmost non-aqueous solvents, e.g. a propylene carbonate (PC)-basedelectrolyte. In contrast, this solvent decomposes on the surface of agraphite-based negative electrode. This reaction, due to the instabilitybetween PC and graphite, causes an exfoliation of the graphite-layeredstructure, which may destroy the electrode structure before any lithiumintercalation can take place. Further, the reaction products (“reducedPC”) may react with the lithium salt of the electrolyte, causing furtherloss of active material. These phenomena have been described by manyscientists, see e.g. Z. Jiang, M. Almagir and K. M. Abraham, J.Electrochem. Soc., Vol. 142, No.2, 333-340 (1995).

Therefore, the unfortunate drawback of most known lithium-ion batterieswith graphite-based negative electrode structures is their poorcompatibility with many electrolyte solvents.

A few organic solvent are stable with respect to graphite, includingethylene carbonate (EC) and dimethyl carbonate (DMC). These solvents,however, suffer from handling difficulties. Since DMC is a volatilesolvent with a low boiling point of 90° C., and since EC has a meltingpoint of 38° C. and therefore is solid at room temperature, theirhandling is difficult. During the addition of the electrolyte to thebattery a large amount of DMC may evaporate, whereas EC may solidify. Interms of handling, solvents with high boiling points, which also havelow melting points, are preferred in these batteries which operatebetween −20° C. and 60° C. Such solvents, including the abovementionedpropylene carbonate, have traditionally been unstable with respect tographite.

In the literature, few examples are given on graphite materials whichcan work with a PC-containing electrolyte.

U.S. Pat. No. 5,643,695 to Valence describes a battery, the firstelectrode of which is a graphite based electrode, characterised in thatthe interlayer spacing of the graphite (d₀₀₂) is in the range 3.35-3.36Å, the crystallite size in the direction of the c-axis (L_(c)) is in therange 100-200 nm, the BET surface area is in the range 6-16 m²/g and atleast 90% of the graphite particles have a size less than 16 μm. Theelectrolyte of the battery configuration is a mixture of EC, PC andoptionally one other solvent, the EC being present in an amount not lessthan the amount of PC. The preferred graphite of this invention isSFG-15 from Lonza, which is an artificial graphite.

In most cases, however, PC-graphite combinations suffer from high lossesof active material due to solvent decomposition and passivating filmformation. In addition, for the above technology based on artificialgraphite, only a rather poor density of the negative electrode structurecan be obtained. The stacking of the “cubic” crystallites of theartificial graphites does not allow densities greater than 0.6 g/cm³.Such low gravimetric density of the negative electrode leads to a ratherlow energy density of the complete battery.

Therefore, there is a need for new lithium secondary batteryconfigurations of applicable solvents and graphite electrodes in orderto facilitate handling, achieve a broad working temperature range, andprovide good chemical and electrochemical stabilities. Suchconfigurations should have high capacity, high energy density, lowinitial loss and flat discharge voltage profile.

It is thus an object of the invention to provide a new type of lithiumsecondary battery of high capacity, energy density and flat dischargeprofile, based on a graphite negative electrode structures, which canwork with a broader selection of PC-containing electrolytes.

Surprisingly it has been found, that a group of electrodes based onnatural flake graphite is compatible with PC-containing electrolytes,thereby forming stable battery configurations. This new type ofelectrode material shows a low initial capacity loss of less than 100mAh/g, i.e. 22% of the initial capacity, a high reversible capacity ofmore than 340 mAh/g, good cyciability and flat discharge voltageprofile. The natural flake graphites allow construction of electrodestructures of densities higher than 0.8 g/cm³, compared to theartificial graphites of an electrode density lower than 0.7 g/cm³ withthe same configuration. The higher gravimetric density of the negativeelectrode allows a higher energy density of the complete battery.

The use of natural graphite is described in the Japanese patentapplication JP 08,298,117 A to Kansai Coke & Chem. Co. Ltd. This patentapplication describes obtaining improved charge/discharge capacity andefficiency by using scalelike natural graphite pulverised by a jet mill.The application, however, does not describe the advantageous combinationof natural graphite and propylene carbonate.

The present invention provides a lithium secondary battery comprising anegative electrode structure, an electrolyte and a positive cathodestructure, said negative electrode structure comprising at least 40% byweight of natural flake graphite having an L_(a)/L_(c)-ratio of at least2, and said electrolyte comprising at least 10% by weight of propylenecarbonate based on the weight of the solvent and salts of theelectrolyte system. Such compositions are compatible with PC-containingelectrolytes, thereby forming stable battery configurations.

It is preferred that the natural flake graphite has L_(a)-values above300 nm and L_(c) values in the range 50-150 nm.

In a preferred embodiment of the invention, the natural flake graphitehas a particle size of less than 10 μm, preferably less than 5 μm.

The positive electrode structure may be a Li-intercalation material,preferably a lithium transition metal oxide, such as LiCoO₂, LiNiO₂,LiMn₂O₄, LiMnO₂, or a mixture thereof, or any other material capable ofdonating Li⁺-ions.

In another preferred embodiment, a carbon black is added to the negativeelectrode structure to ensure good electronic conductivity in thenegative electrode. According to this embodiment the carbon blackpreferably has a low surface area. Most preferably it is a lampblack oran acetylene black.

The carbon black material used according to the invention is present asan additive in electrodes based predominantly on natural flake graphite.The resulted negative electrode has a low and staged voltage profileversus lithium, which is characteristic for lithium intercalation intographite materials.

Thus, in a preferred embodiment of the invention the negative electrodecomposition comprises 40 to 81% by weight of natural flake graphite, 15to 56% by weight of a lampblack, and 4 to 10% by weight of a binder,preferably a polymeric binder.

In another preferred embodiment of the invention the negative electrodecomposition comprises 80 to 93% by weight of natural flake graphite, 3to 16% by weight of an acetylene black, and 4 to 15% by weight of abinder, preferably a polymeric binder.

In a preferred embodiment of the invention the electrolyte comprises asolvent mixture (in addition to PC) of one or more of the followingsolvents:

(a) alicyclic carbonates represented by the following general formula:

 —C(═O)—O—CR₁R₂—[CR₃R₄]_(m)—CR₅R₆—O—,

 wherein each of R₁, R₂, R₃, R₄, R₅ and R₆ independently representshydrogen or a C₁-C₄ alkyl group and m is 0 or 1, with the proviso thatwhen m is 0 and all of R₁, R₂ and R₅ are hydrogen, R₆ is not methyl,preferably ethylene carbonate;

(b) aliphatic carbonates represented by the general formulaR₇[OC(O)]_(p)OR₈, wherein each of R₇ and R₈ independently represents aC₁-C₄ alkyl group, and p is an integer equal to 1 or 2, preferablydimethyl carbonate or diethyl carbonate;

(c) lactones in the form of cyclic esters represented by the generalformula:

—C(═O)—CR₉R₁₀—CR₁₁R₁₂—[CR₁₅R₁₆]_(r)—CR₁₃R₁₄—O—

 wherein each of R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, R₁₅ and R₁₆ independentlyrepresents hydrogen or a C₁₋₂ alkyl group and r is 0 or 1, preferablyγ-valerolactone or γ-butyrolactone;

(d) esters represented by the formula R₁₇[C(O)]OR₁₈[OR₁₉]_(t), whereineach of R₁₇, R₁₈ and R₁₉ independently represents hydrogen or a C₁-C₂alkyl group, and t is 0 or an integer equal to 1 or 2, preferably anacetate, more preferably (2 methoxyethyl)-acetate or ethyl acetate;

(e) glymes represented by the general formula R₂₀O(R₂₁O)_(n)R₂₂, inwhich each of R₂₀ and R₂₂ independently represents a C₁₋₂ alkyl group,R₂₁ is —(CR₂₃R₂₄CR₂₅R₂₆)— wherein each of R₂₃, R₂₄, R₂₅ and R₂₆independently represents hydrogen or a C₁-C₄ alkyl group, and n is aninteger from 2 to 6, preferably 3, R₂₀ and R₂₂ preferably being methylgroups, R₂₃, R₂₄, R₂₅ and R₂₆ preferably being hydrogen or C₁-C₂ alkylgroups, more preferably hydrogen.

Preferably the salt included in the electrolyte is an alkali metal saltor a quaternary ammonium salt of ClO₄—, CF₃SO₃—, AsF₆—, PF₆— or BF₄—, orany mixture of such alkali or ammonium salts, preferably LiAsF₆,LiCF₃SO₃, LiPF₆, LiBF₄, N(Et)₄BF₄ or N(Bu)₄BF₄ or any mixture thereof,more preferably LiPF₆ or LiBF₄.

The present invention is furthermore concerned with a simple andeconomically advantageous method for the production of a negativeelectrode composition for lithium secondary batteries, said electrodecomposition offering improved performance in combination with PC-basedelectrolytes in terms of high capacity, good cyclability and reducedirreversible capacity loss compared to electrodes of thestate-of-the-art.

Thus, according to the invention a method is provided by which a carbonblack and a natural flake graphite are ground in a solvent whichcontains a binder, to produce a uniform, viscous slurry, followed bycoating of the slurry onto a substrate, preferably a metal foilsubstrate, evaporation of the solvent and drying at elevatedtemperature.

The binder is preferably a polymeric binder, more preferably EPDM(ethylene-propylene-diene-polymethylene).

The solvent used in the electrode manufacturing process is preferablyselected among alicyclic compounds, such as cyclohexane.

The following non-limiting examples illustrate production of variousembodiments of the electrode composition according to the invention.

EXAMPLE 1

2.6 g of a natural flake graphite (HPM850 from Asbury, USA) of aparticle size lower than 5 μm, 2.0 g of lamp black and 8.0 g of a 5%solution of EPDM (ethylene-propylene-diene-polymethylene; amorphous,oil-free, of medium saturation, ethylene content of 50%) in cyclohexanewas introduced into a Ø100 mm porcelain mortar. The resulting mixturewas subjected to 30 min. of grinding in order to mix the components andto produce a uniform slurry for coating. During mixing a further 15.0 gof cyclohexane was added. A portion of the slurry was poured onto anickel foil substrate and spread out on the substrate by wirebarcoating, thereby forming a uniform thin layer. The coated layer was keptin air for 20 min., and then dried in an oven at 110° C. for 3 hours.The resulting negative electrode was tested in half cells againstlithium metal, applying a 1M LiPF₆ in PC/EC (50%/50% by weight)electrolyte. In the test the electrodes were charged and dischargedgalvanostatically between 0.01 V and 1.5 V vs. Li/Li⁺. Irreversiblecapacity losses were derived from the first charge-discharge cycle,whereas the reversible capacity was defined as the first dischargecapacity.

The electrode obtained having a composition of 52% by weight of agraphite, 40% by weight of lamp black and 8% by weight of binder,provided a reversible capacity of 345 mAh/g. The initial irreversiblecapacity loss was 100 mAh/g, say about 22% of the initial capacity. Thedischarge-charge cyclability of the electrode produced exceeded 100cycles at 80% of the initial reversible capacity.

The said reversible capacity of 345 mAh/g was obtained at a 1 hourdischarge rate. At a 20 min. 3C discharge rate, the reversible capacitywas 245 mAh/g.

COMPARATIVE EXAMPLE

2.6 g of graphitized carbon (artificial graphite, KS6 from TIMCAL(former Lonza) of Switzerland), 2.0 g of lamp black and 8.0 g of a 5%solution of EPDM (ethylene-propylene-diene-polymethylene; amorphous, oilfree, of medium saturation, ethylene content of 50%) in cyclohexane wasintroduced into a Ø100 mm porcelain mortar. The resulting mixture wassubjected to 30 min. of grinding in order to mix the components and toproduce a uniform slurry for coating. During mixing a further 15.0 g ofcyclohexane was added. A portion of the slurry was poured onto a nickelfoil substrate and spread out on the substrate by wirebar coating,thereby forming a uniform thin layer. The coated layer was kept in airfor 20 min., and then dried in an oven at 110 C for 3 hours. Theresulting negative electrode was tested in half cells against lithiummetal, applying a 1M LiPF₆ in PC/EC (50%/50% by weight) electrolyte. Inthe test the electrodes were charged and discharged galvanostaticallybetween 0.01 V and 1.5 V vs. Li/Li⁺. Irreversible capacity losses werederived from the first charge-discharge cycle, whereas the reversiblecapacity was defined as the first discharge capacity.

The electrode obtained having a composition of 52% by weight ofgraphite, 40% by weight of lamp black and 8% by weight of binder,provided a reversible capacity of 322 mAh/g. The initial irreversiblecapacity loss was 1080 mAh/g, say about 77% of the initial capacity.

EXAMPLE 2

A series of natural graphite materials with different mean particle size(3-25 μm) were made into electrodes following the above procedure. Theywere tested in electrochemical cells using lithium metal as counterelectrode and 1M LiPF₆ in ethylene carbonate/dimethyl carbonate (EC/DMC)and 1M LiPF₆ in ethylene carbonate/propylene carbonate (EC/PC) aselectrolyte, respectively. With EC/DMC it was found, that the initialcapacity loss decreased as the mean particle size was increased. Withthe EC/PC-electrolyte, the initial loss increased dramatically with themean particle size. When the mean particle size was lower than 10 μm,however, the loss with EC/PC was just as low as with EC/DMC.

EXAMPLE 3

Electrochemical cells were prepared from laminates of negative electrodecomposition of the above Example 1, laminates of a lithium manganeseoxide spinel positive electrode (prepared from lithium carbonate andmanganese dioxide by a solid state reaction at 800° C.) coated on analuminium foil, and a 1M LiPF₆ in PC/EC (50%/50% by weight) electrolytesandwiched between said laminates. The negative electrode structures ofsuch cells displayed gravimetric and charge densities as high as 0.8g/cc and 240 mAh/cm³ respectively and provided a battery energy densityof 180 Wh/l.

What is claimed is:
 1. A lithium secondary battery comprising a negativeelectrode structure, an electrolyte and a positive cathode structure,said negative electrode structure comprising at least 40% by weight ofnatural flake graphite having an L_(a)-value of at least 300 nm and/oran L_(c)-value in the range 50-150 nm, and an L_(a)/L_(c)-ratio of atleast 2, and said electrolyte comprising at least 10% by weight ofpropylene carbonate based on the weight of the solvent and salts of theelectrolyte system.
 2. A secondary battery according to claim 1, inwhich the natural flake graphite has a particle size of 10 μm or less,preferably 5 μm or less.
 3. A secondary battery according to claim 1, inwhich the negative electrode structure comprises at least 3% by weightof a carbon black.
 4. A secondary battery according to claim 3, in whichsaid carbon black is an acetylene black.
 5. A secondary batteryaccording to claim 4, in which the negative electrode structurecomprises 80 to 93% by weight of natural flake graphite, 3 to 16% byweight of an acetylene black, and 4 to 15% by weight of a polymericbinder.
 6. A secondary battery according to claim 1, in which theelectrolyte comprises at least 40 percent by weight of propylenecarbonate.
 7. A secondary batter according to claim 1, wherein thesolvent in addition to propylene carbonate, comprises one or more of thefollowing solvents (a) to (e): (a) alicyclic carbonates represented bythe following general formula: —C(═O)—O—CR₁R₂—[CR₃R₄]_(m)—CR₅R₆—O—, wherein each of R₁, R₂, R₃, R₄, R₅ and R₆ independently representshydrogen or a C₁-C₄ alkyl group and m is 0 or 1, with the proviso thatwhen m is 0 and all of R₁, R₂ and R₅ are hydrogen, and R₆ is other thanmethyl carbonate; (b) aliphatic carbonates represented by the generalformula R₇[OC(O)]_(p)OR₈, wherein each of R₇ and R₈ independentlyrepresents a C₁-C₄ alkyl group, and p is an integer equal to 1 or 2, andis one of dimethyl carbonate or diethel carbonate; (c) lactones in theform of cyclic esters represented by the general formula:—C(═O)—CR₉R₁₀—CR₁₁R₁₂—[CR₁₅R₁₆]_(r)—CR₁₃R₁₄—O—  wherein each of R₉, R₁₀,R₁₁, R₁₂, R₁₃, R₁₄, R₁₅ and R₁₆ independently represents hydrogen or aC₁₋₂ alkyl group and r is 0 or 1 and is one of γ-valerolactone orγ-butyrolactone; (d) esters represented by the formulaR₁₇[C(O)]OR₁₈[OR₁₉]_(t), wherein each of R₁₇, R₁₈ and R₁₉ independentlyrepresents hydrogen or a C₁-C₂ alkyl group, and t is 0 or an integerequal to 1 or 2, and an acetate; (e) glymes represented by the generalformula R₂₀O(R₂₁O)_(n)R₂₂, in which each of R₂₀ and R₂₁ independentlyrepresents a C₁₋₂ alkyl group, R₂₁ is —(CR₂₃R₂₄CR₂₅R₂₆)— wherein R₂₃,R₂₄, R₂₅ and R₂₆ each independently represents hydrogen or a C₁-C₄ alkylgroup, and n is an integer from 2 to 6, R₂₀ and R₂₂ are methyl groups,R₂₃, R₂₄, R₂₅ and R₂₆ are one of hydrogen or C₁-C₂ alkyl groups.
 8. Asecondary battery according to claim 1, in which the salt of theelectrolyte is one of an alkali metal salt or a quaternary ammonium saltselected from the group consisting of ClO₄—, CF₃SO₃—, AsF₆—, PF₆—, orBF₄—, or any mixture of such alkali or ammonium salts, selected from thegroup consisting of LiAsF₆, LiCF₃SO₃, LiPF₆, LiBF₄, N(Et)₄BF₄ orN(Bu)₄BF₄ or any mixture thereof.
 9. A secondary battery according toclaim 1, in which the positive electrode structure is a Li-intercalationmaterial selected from the group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄Or LiMnO₂ or a mixture thereof.
 10. A method for the production of asecondary battery according to claim 1 comprising the steps of:preparing the negative electrode composition by: grinding the graphiteand any carbon black in a solvent containing a binder until a uniform,viscous slurry is obtained; coating the slurry onto a substrate;evaporating the solvent and drying; preparing the electrolyte by: mixingthe solvents in the case where the electrolyte comprises more than onesolvent, the final solvent comprising at least 10% by weight ofpropylene carbonate; dissolving alkali metal or ammonium salt(s) in thesolvent(s) to provide a propylene carbonate-based electrolyte; preparingthe positive electrode composition by: grinding positive electrodematerial in a solvent containing a binder until a uniform, viscousslurry is obtained; coating the slurry onto a substrate, evaporating thesolvent and drying; and sandwiching said electrolyte between theelectrode laminates to form the battery.
 11. A lithium secondary batterycomprising a negative electrode structure, an electrolyte and a positivecathode structure, said negative electrode structure comprising at least40% by weight of natural flake graphite having an L_(a)/L_(c)-ratio ofat least 2, and said electrolyte comDrising at least 10% by weight ofpropylene carbonate based on the weight of the solvent and salts of theelectrolyte system, wherein the negative electrode structure comprisesat least 3% by weight of a carbon black, and wherein said carbon blackis a lampblack.
 12. A secondary battery according to claim 5, in whichthe negative electrode structure comprises 40 to 81% by weight ofnatural flake graphite, 15 to 56% by weight of a lampblack, and 4 to 10%by weight of a binder, preferably a polymeric binder.